Gas barrier films and packaging materials using the same have been well known. For example, as a material having a gas barrier property, an aluminum foil has been known. Since pinholes easily take place in this aluminum foil alone, which cannot be used except for a special usage, it is used almost as an intermediate layer in a laminated film. However, there are environment problems in recent years, as for a packaging material, there have been increasingly strong demands for reduction of volume by making it thinner and simplifying a laminate, so there is a tendency that an aluminum foil does not tend to be used as a packaging material. Since a laminated film using an aluminum foil is not transparent, when it is used as a packaging material, contents cannot be seen, so it is difficult to judge whether it is surely heat sealed or not.
Since thermoplastic resin films such as polyester type resin film and polyamide type resin film are excellent in strength, transparency and formability, they have been used as a packaging material in wide applications. However, since these thermoplastic resin films have high permeability to gases such as oxygen and water vapor, when they are used as package of products such as ordinary foods, retort-treatment foods, drugs and medicines, there is a case that the products are degraded and deteriorated due to storage for a long period of time.
Therefore, for a packaging material required for a gas barrier property, there has been often used a thermoplastic resin film that polyvinylidene chloride (hereinafter PVDC) emulsion or the like is coated on a polyolefin type resin film, polyamide type resin film, polyester type resin film such as polyethylene terephthalate. The thermoplastic resin film that a PVDC layer has been formed by coating exerts a high oxygen-barrier property not only in a low humidity but also in a high humidity, and also has high barrier properties to water vapor. However, this thermoplastic resin film that a PVDC layer has been coated has a fear of generating chlorine gas derived and dioxins from chlorine in PVDC in burning in waste treatment. Therefore, because there is a fear adversely affecting environments and human bodies to great extent, it has been strongly desired to shift to other materials.
As a gas barrier material not containing chlorine, polyvinyl alcohol (hereinafter PVA) film, and a coated film that PVA or ethylene-vinyl alcohol copolymer (hereinafter EVOH) is coated on a thermoplastic resin film are most well known. PVA and EVOH are very excellent in oxygen gas barrier property under dry environment. However, they have problems that the barrier property depends very largely on humidity and markedly deteriorates under high humidity conditions, and they have no water vapor barrier property and are easily degraded in hot water.
Regarding such problems, as a polymer that the lowering of gas barrier property of PVA and EVOH under high humidity is improved, a resin composition containing a partially neutralized material of PVA with polyacrylic acid or polymethacrylic acid is proposed (Patent document 1). A composition containing a vinyl polymer consisting mainly of PVA and polyitaconic acid, and a metal compound is also proposed (Patent document 2).
There is also proposed a vapor deposition film that on one surface of a thermoplastic resin film such as polyester type resin film, using a physical vapor growth method such as vacuum deposition method, for example, a deposited film of metal such as aluminum, or a deposited film of inorganic oxide such as aluminum oxide and silicon oxide is provided (Patent documents 3 to 5). However, the gas barrier property was unstable and insufficient largely depending on surface roughness and thermal shrinkage of the thermoplastic resin film as a substrate.
As a technique to make up for the drawbacks described above, there is proposed a gas barrier film that a polyurethane type polymer layer is provided on a thermoplastic resin film, and a metal or metal oxide-covering layer is formed thereon (Patent document 6).
Patent document 1: Japanese Unexamined Patent Publication No. 10-237180 (1998) (paragraph numbers [0060] to [0065])
Patent document 2: Japanese Unexamined Patent Publication No. 2004-35833 (paragraph numbers [0061] to [0066])
Patent document 3: Japanese Unexamined Patent Publication No. 8-269689 (1996) (paragraph numbers [0004] to [0005])
Patent document 4: Japanese Unexamined Patent Publication No. 2000-185374 (paragraph numbers [0016] to [0018])
Patent document 5: Japanese Unexamined Patent Publication No. 11-322982 (1999) (paragraph numbers [0007] to [0033])
Patent document 6: Japanese Unexamined Patent Publication No. 2001-10003 (paragraph numbers [0035] to [0041])